Thirty years of unique data reveal what really kills coral reefs

Coral reefs are considered one of the most endangered ecosystems on the planet and are dying at an alarming rate around the world. Scientists attribute coral bleaching and mass mortality to a number of environmental stressors, particularly warming water temperatures due to climate change.

A study published in the international journal Marine biology, reveals what really kills coral reefs. With unique data on Looe Key Reef in the lower Florida for 30 years, researchers and collaborators at the Florida Atlantic University's Harbor Institute Oceanographic Institute have discovered that the problem of coral bleaching is not just about due to global warming, but also to the planet. simultaneously enriched with reactive nitrogen from multiple sources.

Poorly treated sewage, fertilizers and the upper soil layer cause nitrogen levels to rise, resulting in phosphorus deprivation in corals, reducing their "bleaching" temperature threshold. These coral reefs died well before the impact of rising water temperatures. This study represents the longest record of concentrations of reactive nutrients and algae for coral reefs in the world.

"Our results provide compelling evidence that nitrogen loading from the Florida Keys and the larger Everglades ecosystem, caused by humans, rather than global warming, is the major contributor to coral reef degradation in the world. the Looe Key sanctuary preservation area during our long-term study, "said Brian Lapointe, Ph.D., lead author and research professor at the FAU Harbor Branch.

One of the main conclusions of the study is that nutrient runoff to soil has increased the nitrogen: phosphorus (N: P) ratio in reef algae, indicating an increasing degree of phosphorus limitation that may metabolic stress and possibly starvation in corals. Reactive nitrogen concentrations are above the critical thresholds set by ecosystems for the Florida Keys, as are phytoplankton levels for offshore reefs, as evidenced by the presence of macroalgae and other blooms. Harmful algae due to excessive levels of nutrients.

The researchers collected data from 1984 to 2014 and samples of seawater during dry and wet seasons. Lapointe and collaborators from the University of Georgia and the University of South Florida also monitored living coral and collected numerous species of algae (macroalgae) for analysis of nutrients of the tissues. They monitored the salinity of seawater, temperature, and nutrient gradients between the Everglades and Looe Key. They wanted to better understand how nitrogen was flowing from the Everglades downstream from the coral reefs of the Florida Keys National Marine Sanctuary, which now has the lowest coral coverage of any reef in the Caribbean.

The data revealed that live coral cover in the Looe Key Sanctuary Preservation Area has increased from almost 33% in 1984 to less than 6% in 2008. The annual rate of coral loss has varied over the past decade. 39 study, but increased from 1985 to 1987 and from 1996 to 1999 heavy rainfall and the increase in water deliveries of the Everglades. Between 1991 and 1995, the significant increase in Everglades runoff and heavy precipitation resulted in increased levels of reactive nitrogen and phytoplankton at Looe Key relative to levels known to stress and cause death to coral reefs. Despite the reduction in the flow of the Everglades, the water quality has not yet recovered its level of the 1980s.

Nitrogen load on the coast is expected to rise by 19% worldwide simply because of changes in precipitation due to climate change, suggesting the need for urgent management measures to avoid further degradation.

"The future success of the Everglades Global Restoration Plan will be based on recognizing the hydrological and nitrogen links between the Everglades, Florida Bay and Florida Keys," said Lapointe. "The good news is that we can do something about the nitrogen problem, such as improving wastewater treatment, reducing fertilizer inputs and increasing stormwater storage and treatment on the land. Florida continent. "

The impact of local nitrogen contributions from sewage treatment water serving 76,000 residents year-round and about 3.8 million tourists annually is currently being mitigated by completion Centralized wastewater collection and sewage treatment facilities and nutrient removal facilities throughout the Florida Keys. .

According to the Florida Keys National Marine Sanctuary, coral reef-related oceans activities generate more than $ 8.5 billion a year and create 70,400 jobs in Southeast Florida's local economy.

"The coral reefs of Bonaire in the Caribbean Netherlands are an excellent example of effective reduction of nitrogen pollution.These reefs begin to recover after the construction of a new wastewater treatment plant. in 2011, which significantly reduced the nitrogen load of septic tanks, "said Lapointe.

The co-authors of the study are Rachel A. Brewton and Laura W. Herren from the Harbor Branch of the FAU; James W. Porter, Ph.D., emeritus professor of ecology at the University of Georgia; and Chuanmin Hu, Ph.D., of the College of Marine Science at the University of South Florida.

"Quoting climate change as the exclusive cause of the disappearance of coral reefs worldwide misses the crucial point that water quality also plays a role," Porter said. "Although riparian coral reef communities have few means to fight global warming, they can do anything to reduce nitrogen runoff." Our study shows that the fight for the preservation of coral reefs requires local action , not just global ".